In recent years, there has been an increasing interest in the use of ceramics for structural applications historically served by metals. The impetus for this interest has been the superiority of ceramics with respect to certain properties, such as corrosion resistance, hardness, modulus of elasticity, and refractory capabilities, when compared with metals.
Current efforts at producing higher strength, more reliable, and tougher ceramic articles are largely focused upon (1) the development of improved processing methods for monolithic ceramics and (2) the development of new material compositions, notably ceramic matrix composites. A composite structure is one which comprises a heterogeneous material, body or article made of two or more different materials which are intimately combined in order to attain desired properties of the composite. For example, two different materials may be intimately combined by embedding one in a matrix of the other. A ceramic matrix composite structure typically comprises a ceramic matrix which incorporates one or more diverse types of filler material such as particulates, fibers, rods, and the like.
There are several known limitations or difficulties in substituting ceramics for metals, such as scaling versatility, capability to produce complex shapes, satisfying the properties required for the end use application, and costs. Several copending patent applications, and Issued Patents, assigned to the same owner as this application (hereinafter sometimes referred to as "Commonly Owned Patent Applications and Patents"), overcome these limitations or difficulties and provide novel methods for reliably producing ceramic materials, including composites. The method is disclosed generically in Commonly Owned U.S. Pat. No. 4,713,360, issued on Dec. 15, 1987 in the names of Marc S. Newkirk et al. and entitled "Novel Ceramic Materials and Methods for Making Same". This Patent discloses a method of producing self-supporting ceramic bodies grown as the oxidation reaction product of a molten parent precursor metal which is reacted with a vapor-phase oxidant to form an oxidation reaction product. Molten metal migrates through the formed oxidation reaction product to react with the oxidant thereby continuously developing a ceramic polycrystalline body which can, if desired, include an interconnected metallic component. The process may be enhanced by the use of one or more dopants alloyed with the parent metal. For example, in the case of oxidizing aluminum in air, it is desirable to alloy magnesium and silicon with the aluminum to produce alpha-alumina ceramic structures. This method was improved upon by the application of dopant materials to the surface of the parent metal, as described in Commonly Owned U.S. Pat. No. 4,853,352, issued on Aug. 1, 1989, in the names of Marc S. Newkirk et al. and entitled "Method of Making Self-Supporting Ceramic Materials and Materials Made Thereby", which issued from U.S. patent application Ser. No. 220,935, filed Jun. 23, 1988, which is a continuation-in-part of U.S. patent application Ser. No. 822,999, filed Jan. 27, 1986 (and now abandoned). which is a continuation-in-part of U.S. patent application Ser. No. 776,965, filed Sep. 17, 1985 (and now abandoned), which is a continuation-in-part of U.S. application Ser. No. 747,788, filed Jun. 25, 1985 (and now abandoned), which is a continuation-in-part of U.S. patent application Ser. No. 632,636, filed Jul. 20, 1984 (and now abandoned), all in the names of Marc S. Newkirk et al. and entitled "Methods of Making Self-Supporting Ceramic Materials".
A similar oxidation phenomenon was utilized to grow oxidation reaction product into a filler material to produce ceramic composite bodies, as described in Commonly Owned U.S. Pat. No. 4,916,113, which issued on Apr. 10, 1990, from U.S. patent application Ser. No. 265,835, filed Nov. 1, 1988, both of which are entitled "Methods of Making Composite Ceramic Articles." patent application Ser. No. 265,835 is a continuation of U.S. patent application Ser. No. 819,397, (now U.S. Pat. No. 4,851,375), entitled "Methods of Making Composite Ceramic Articles Having Embedded Filler", which was a continuation-in-part of U.S. patent application Ser. No. 697,876, filed Feb. 4, 1985 (and now abandoned), in the names of Marc S. Newkirk et al. and entitled "Composite Ceramic Articles and Methods of Making Same". Specifically, these applications and patents disclose novel methods for producing a self-supporting ceramic composite by growing an oxidation reaction product from a parent metal into a permeable mass of filler (e.g., a silicon carbide particulate filler), thereby infiltrating the filler with a ceramic matrix. The resulting composite, however, has no defined or predetermined geometry, shape, or configuration.
A method for producing ceramic composite bodies having a predetermined geometry or shape is disclosed in Commonly Owned U.S. Pat. No. 5,017,526, which issued on May 21, 1991, from U.S. patent application Ser. No. 338,471, filed Apr. 14, 1989, which is a continuation of U.S. patent application Ser. No. 861,025, filed May 8, 1986, and now abandoned, all in the names of Marc S. Newkirk et al. and entitled "Shaped Ceramic Composites and Methods of Making the Same" (a corresponding EPO application was published in the EPO on Nov. 11, 1987, as EPO Publication No. 0245192). In accordance with the method in this U.S. Pat. No. 4,923,832, which issued on May 8, 1990, from the developing oxidation reaction product infiltrates a permeable preform of filler material (e.g., a silicon carbide preform material) in a direction towards a defined surface boundary.
It was discovered that high shape fidelity is more readily achieved by providing the preform with a barrier means, as disclosed in Commonly Owned U.S. patent application Ser. No. 861,024, filed May 8, 1986, in the names of Marc S. Newkirk et al. (a corresponding EPO application was published in the EPO on Nov. 11, 1987, as EPO Publication No. 0245193). This method produces shaped self-supporting ceramic bodies, including shaped ceramic composites, by growing the oxidation reaction product of a parent metal to a barrier means spaced from the metal for establishing a boundary or surface. Ceramic composites having a cavity with an interior geometry inversely replicating the shape of a positive mold or pattern are disclosed in Commonly Owned U.S. patent application Ser. No. 329,794 now allowed, filed Mar. 28, 1989, which is a divisional application of U.S. Pat. No. 4,828,785, entitled "Inverse Shape Replication Method of Making Ceramic Composite Articles" which issued May 9, 1989, both in the names of Marc S. Newkirk et al, and in Commonly Owned U.S. Pat. No. 4,859,640, which issued on Aug. 22, 1989, from U.S. patent application Ser. No. 896,157, filed Aug. 13, 1986, in the name of Marc S. Newkirk, and entitled "Method of Making Ceramic Composite Articles with Shape Replicated Surfaces and Articles Obtained Thereby".
The feeding of additional molten parent metal from a reservoir has been successfully utilized to produce thick ceramic matrix composite structures. Particularly, as disclosed in and Commonly Owned U.S. Pat. No. 4,918,034, which issued on Apr. 17, 1990, from U.S. patent application Ser. No. 168,358, filed Mar. 15, 1988, which is a continuation-in-part of U.S. Pat. No. 4,900,699, which issued on Feb. 13, 1990, from U.S. patent application Ser. No. 908,067, filed Sep. 16, 1986, both in the names of Marc S. Newkirk et al. and entitled "Reservoir Feed Method of Making Ceramic Composite Structures and Structures Made Thereby" (a corresponding EPO application was published in the EPO on Mar. 30, 1988, as EPO Publication No. 0262075), the reservoir feed method has been successfully applied to form ceramic matrix composite structures. According to the method of this Newkirk et al. invention, the ceramic or ceramic composite body which is produced comprises a self-supporting ceramic composite structure which includes a ceramic matrix obtained by the oxidation reaction of a parent metal with an oxidant to form a polycrystalline material. In conducting the process, a body of the parent metal and a permeable filler are oriented relative to each other so that formation of the oxidation reaction product will occur in a direction toward and into the filler. The parent metal is described as being present as a first source and as a reservoir, the reservoir of metal communicating with the first source due to, for example, gravity flow. The first source of molten parent metal reacts with the oxidant to begin the formation of the oxidation reaction product. As the first source of molten parent metal is consumed, it is replenished, preferably by a continuous means, from the reservoir of parent metal as the oxidation reaction product continues to be produced and infiltrates the filler. Thus, the reservoir assures that ample parent metal will be available to continue the process until the oxidation reaction product has grown to a desired extent.
Moreover, U.S. patent application Ser. No. 568,618 (now allowed), filed Aug. 16, 1990, which is a continuation of U.S. patent application Ser. No. 269,152, filed Nov. 9, 1988, both in the names of Robert C. Kantner et al. and entitled "Method for in situ Tailoring the Metallic Component of Ceramic Articles and Articles Made Thereby", which is a continuation of Commonly Owned U.S. patent application Ser. No. 152,518, filed on Feb. 5, 1988, in the same names and having the same title; which was a continuation-in-part of U.S. patent application Ser. No. 908,454, filed Sep. 17, 1986, in the names of Marc S. Newkirk et al. and having the same title. U.S. patent application Ser. No. 152,518, issued on Apr. 4, 1989, as U.S. Pat. No. 4,818,734, in the names of Robert C. Kantner et al. Each of these applications and Patent discloses a method for tailoring the constituency of the metallic component (both isolated and interconnected) of ceramic and ceramic matrix composite bodies during formation thereof to impart one or more desirable characteristics to the resulting body. Thus, desired performance characteristics for the ceramic or ceramic composite body are advantageously achieved by incorporating the desired metallic component in situ, rather than from an extrinsic source, or by post-forming techniques.
As discussed in these Commonly Owned Patent Applications and Patents, novel polycrystalline ceramic materials or polycrystalline ceramic composite materials are produced by the oxidation reaction between a parent metal and an oxidant (e.g., a solid, liquid and/or a gas). In accordance with the generic process disclosed in these Commonly Owned Patent Applications and Patents, a parent metal (e.g., aluminum) is heated to an elevated temperature above its melting point but below the melting point of the oxidation reaction product to form a body of molten parent metal which reacts upon contact with an oxidant to form the oxidation reaction product. At this temperature, the oxidation reaction product, or at least a portion thereof, is in contact with and extends between the body of molten parent metal and the oxidant, and molten metal is drawn or transported through the formed oxidation reaction product and towards the oxidant. The transported molten metal forms additional fresh oxidation reaction product upon contact with the oxidant, at the surface of the previously formed oxidation reaction product. As the process continues, additional metal is transported through this formation of polycrystalline oxidation reaction product thereby continually "growing" a ceramic structure of interconnected crystallites. The resulting ceramic body may contain metallic constituents, such as non-oxidized constituents of the parent metal, and/or voids. Oxidation is used in its broad sense in all of the Commonly Owned Patent Applications and Patents discussed in this application, and refers to the loss or sharing of electrons by a metal to an oxidant which may be one or more elements and/or compounds. Accordingly, elements other than oxygen may serve as an oxidant.
In certain cases, the parent metal may require the presence of one or more dopants in order to influence favorably or to facilitate growth of the oxidation reaction product. Such dopants may at least partially alloy with the parent metal at some point during or prior to growth of the oxidation reaction product. For example, in the case of aluminum as the parent metal and air as the oxidant, dopants such as magnesium and silicon, to name but two of a larger class of dopant materials, can be alloyed with aluminum, and the created growth alloy is utilized as the parent metal. The resulting oxidation reaction product of such a growth alloy comprises alumina, typically alpha-alumina.
Novel ceramic composite structures and methods of making the same are also disclosed and claimed in certain of the aforesaid Commonly Owned Ceramic Matrix Patent Applications and Patents which utilize the oxidation reaction to produce ceramic composite structures comprising a substantially inert filler (note: in some cases it may be desirable to use a reactive filler, e.g., a filler which is at least partially reactive with the advancing oxidation reaction product and/or parent metal) infiltrated by the polycrystalline ceramic matrix. As a first step, a parent metal is positioned adjacent to a mass of permeable filler (or a preform) which can be shaped and/or treated to be self-supporting. The parent metal is then heated to form a body of molten parent metal which is reacted with an oxidant, as described above, to form an oxidation reaction product. As the oxidation reaction product grows and infiltrates the adjacent filler material, molten parent metal is drawn through the previously formed oxidation reaction product within the mass of filler and reacts with the oxidant to form additional fresh oxidation reaction product at the surface of the previously formed oxidation reaction product, as described above. The resulting growth of oxidation reaction product infiltrates or embeds the filler and results in the formation of a ceramic composite structure comprising a polycrystalline ceramic matrix embedding the filler. As also discussed above, the filler (or preform) may utilize a barrier means to establish a boundary or surface for the ceramic composite structure.
Moreover, methods of forming novel ceramic and ceramic composite structures with modified surfaces is discussed in several other Commonly Owned Patent Applications and Patents. U.S. patent application Ser. No. 587,593, filed Sep. 24, 1990, which is a continuation of U.S. patent application Ser. No. 308,889, filed Feb. 9, 1989, which is a continuation of U.S. application Ser. No. 908,117 (now U.S. Pat. No. 4,837,232, which issued Jun. 6, 1989), all in the names of Stanley Luszcz et al., and entitled "Dense Skin Ceramic Structure and Method of Making the Same", discloses methods for producing self-supporting ceramic structures comprising a polycrystalline material having a first region and a terminal region, the terminal region differing from the first region in at least one of composition and microstructure due to the claimed method. Particularly, the method of forming the first region of oxidation reaction product is carried out as discussed above in the Commonly Owned Patent Applications and Patent. The terminal region of oxidation reaction product is formed by attenuating, or discontinuing, the transport of molten parent metal from the formed body, and thereafter the reacting is resumed within the temperature range for a time sufficient to transport at least a portion of the interconnected molten parent metal from said first region towards a surface, thereby forming additional oxidation reaction product on the surface of the first region as the newly formed terminal region. In a specific embodiment of the claimed invention, a self-supporting ceramic or ceramic composite structure formed by the claimed method comprises a polycrystalline material having a first region and a terminal region which is integral with the first region. In another aspect of the claimed invention, the terminal region comprises an outer skin and the first region comprises a substrate surmounted by the terminal region, the terminal region having a finer microstructure than the first region. In another aspect of the claimed invention, the first region or a part thereof may be shaped prior to carrying out the second stage reacting on it, and a barrier means may be used to limit and define the extent of formation of the first region and/or the terminal region.
Moreover, a method for forming a ceramic or ceramic composite structure comprising a porous core with a dense surface layer is disclosed in U.S. Pat. No. 5,015,610, which issued on May 14, 1991, from U.S. patent application Ser. No. 414,183, filed Sep. 28, 1989, which is a divisional application of U.S. patent application Ser. No. 908,119, filed Sep. 16, 1986 (now U.S. Pat. No. 4,956,137 which issued on Sep. 11, 1990, both in the name of Ratnesh K. Dwivedi, and entitled "Porous Ceramic Composite with Dense Surface" (a corresponding EPO application was published in the EPO on Mar. 23, 1988, as EPO Publication No. 0261050). The claimed invention relates to forming a self-supporting ceramic composite article by incorporating particulate parent metal within a porous preform, the volume percent of parent metal being sufficient to form a volume of oxidation reaction product which exceeds the total spatial volume available within the preform. The particulate parent metal is made molten and thereafter reacted with an oxidant to form oxidation reaction product, as discussed above. The oxidation reaction product grows into and through the preform, thus transporting molten parent metal towards the oxidant to at least one surface of the preform, wherein a dense surface layer of oxidation reaction product is formed on said at least one surface. Due to the oxidation of the parent metal and the subsequent transport of molten parent metal within the formed oxidation reaction product, porosity is created within the preform, thus resulting in a porous core surrounded by a dense skin.
A method for post-treating formed ceramic and ceramic composite bodies to produce self-supporting ceramic bodies containing or having incorporated therein a second polycrystalline ceramic component is disclosed in U.S. patent application Ser. No. 681,467, filed Apr. 4, 1991, which is a continuation of U.S. patent application Ser. No. 272,514, filed Nov. 16, 1988, which is a continuation of U.S. application Ser. No. 908,458, filed Sep. 17, 1986 (now issued as U.S. Pat. No. 4,806,508), all in the names of Ratnesh K. Dwivedi et al. and entitled "Modified Ceramic Structures and Methods of Making the Same". Specifically, the second ceramic component formed by the method of this invention is sufficient to alter, modify or contribute to the properties of the ceramic body formed originally. In accordance with the method of the invention, the original ceramic body is grown by an oxidation reaction of a parent metal with an oxidant, wherein molten parent metal is transported continuously through previously formed oxidation reaction product, as discussed above in the Commonly Owned Patent Applications and Patents. The ceramic body may include interconnected porosity, due to, for example, complete reaction (i.e., exhaustion) of interconnected metal which was transported during growth of oxidation reaction product. If such interconnected parent metal phase formed during growth of oxidation reaction product is not fully oxidized to form porosity within the body, such parent metal may be removed by a post-oxidation treatment such as acid leaching. Further, the porosity in the formed body is at least partially open or accessible or rendered accessible from at least one external surface of the body. A second ceramic material, or a precursor thereof, is then contacted with the ceramic body at the accessible surface(s) of the formed body so as to infiltrate or impregnate at least a portion of the interconnected porosity in the formed body.
Self-supporting ceramic composites formed by the direct oxidation of a parent metal into a permeable mass of filler material may be subjected to a post-formation heating step to remove or oxidize residual non-oxidized metallic constituents of the formed composites. Specifically, U.S. patent application Ser. No. 414,198, filed Sep. 28, 1989, which is a divisional application of U.S. patent application Ser. No. 002,048, filed Jan. 12, 1987 (and now issued as U.S. Pat. No. 4,874,569), both in the names of Jack A. Kuszyk et al. and entitled "Ceramic Composite and Methods of Making the Same" discloses ceramic or ceramic composite bodies obtained by the oxidation of a molten parent metal comprising an aluminum alloy to produce a polycrystalline material comprising the oxidation reaction product of the parent metal with at least one oxidant which embeds at least one filler material, thereby forming the ceramic composite body. In a subsequent step, the formed composite body is heated in at least one environment selected from the group consisting of an oxygen-containing atmosphere, an inert atmosphere and a vacuum to a second temperature above the temperature at which the oxidation reaction product was formed, but below the melting point of the oxidation reaction product, to remove or oxidize at least a substantial portion of the residual non-oxidized metallic constituents of the parent metal without substantial formation of oxidation reaction product beyond the defined surface boundary, thereby producing a self-supporting ceramic composite.
Commonly owned U.S. patent application Ser. No. 681,467, filed Apr. 4, 1991, which is a continuation of U.S. patent application Ser. No. 415,090, filed Sep. 29, 1989, which is a divisional application of U.S. Pat. No. 4,957,779, which issued on Sep. 18, 1990, from U.S. patent application Ser. No. 157,432, now U.S. Pat. No. 4,957,779 filed Feb. 18, 1988, all in the names of Virgil Irick, Jr. et al. and entitled "A Method for Producing A Protective Layer on a Ceramic Body and A Method of Using A Ceramic Body" discloses a method for producing a protective layer on a ceramic or a ceramic composite body and a method of using the formed ceramic or ceramic composite body in an environment which causes the body to react with gaseous species to form a protective layer. Specifically, ceramic and ceramic composite bodies formed by the methods of the Commonly-Owned Patent Applications and Patents discussed above are exposed to an environment which causes a protective layer to be formed on at least a portion of an external surface of the ceramic composite body. In a preferred embodiment, growth of the above-discussed ceramic composite body may be changed by modifying the oxidant to which the parent metal is exposed. Specifically, the oxidant, whether a solid, liquid or vapor-phase oxidant, or any combination thereof, can be modified to result in the formation of the aforementioned protective layer. Alternatively, in a second embodiment of the invention, the above-described ceramic or ceramic composite body may be completely formed, and, in a subsequent step, subjected to a different oxidant (e.g., different from the oxidant employed to form the ceramic or ceramic composite body).
Thus, the aforesaid Commonly Owned Ceramic Matrix Patents and Patent Applications describe the production of oxidation reaction products which are readily grown to desired sizes and thicknesses and which may be modified to produce ceramic bodies heretofore believed to be difficult, if not impossible, to achieve with conventional ceramic processing techniques.
The entire disclosures of all of the foregoing Commonly Owned Ceramic Matrix Patents and Patent Applications are expressly incorporated herein by reference.